A mass spectrometer is often used with a gas chromatograph or a liquid chromatograph. When used with a gas chromatograph, the mass spectrometer is used as the detector. In such a gas chromatograph, a sample containing plural components is made to pass through a gas chromatograph column, where the components are separated with respect to time while the sample passes through the column, and every one of the separated components is introduced via an interface to the mass spectrometer. In the interface, molecules of the component gas are ionized, and the ions are separated by the mass spectrometer with respect to their mass to charge ratios.
In the gas chromatograph section, the sample gas is carried by a carrier gas in the column, and a separator is provided in the interface to separate the carrier gas from the sample, as described in paragraph [0003] and FIG. 8 of the Japanese Unexamined Patent Publication No. 2002-228637. For the carrier gas, the helium gas is normally used. Since the amount of carrier gas is far greater than that of the sample gas, the carrier gas cannot be completely separated, and some part of it reaches an ionizer of the mass spectrometer, where the amount of the carrier gas reaching the ionizer is still far greater than that of component gases of the sample. Though normally it is difficult to ionize the helium gas, it can be ionized when the electron accelerating voltage is raised to increase the ion producing efficiency in the electron impact (EI) ionization method.
Since, as described above, the amount of the carrier helium gas reaching the ionizer is far greater than that of the components of the sample, the amount of ionized carrier helium gas is still larger than that of the sample component ions, so that the detection signal of the sample components is hidden by the detection signal of, or noise by, the carrier helium ions if no measures are taken. The problem is apparent when light elements (for example, lithium, beryllium, boron, etc.) having masses close to helium are to be analyzed. Though it is possible to suppress ionization of helium atoms by decreasing the electron accelerating voltage in the ionizer, the production of ions of sample components is also suppressed and the sensitivity of the mass analysis deteriorates. This makes the analysis of minute (or trace) components difficult.
When ions having mass to charge ratios far larger than that of helium ions are analyzed, they are adequately separated from the helium ions by a mass separator such as the quadrupole mass filter, and the detection signal of such heavy ions would not be hidden by the detection signal (or noise) of helium ions. But the amount of helium ions is so large that, even if the fraction of helium ions passing through the mass separator is small, the number of helium ions entering the detector is still large compared to that of the sample ions. This makes a considerable background noise in the detection of object ions.
Neutral particles such as helium atoms (molecules) are conventionally eliminated by an ion optical system placed before the mass separator. In the mass spectrometer described in the Japanese Unexamined Patent Publication No. 2000-149865 (which has matured to patent No. 3379485), the outlet axis of the heated capillary and the inlet axis of the subsequent skimmer are designed to be displaced: ions are guided from the exit of the heated capillary to the entrance of the skimmer by an electric field formed by an ion optical system provided between them, while neutral particles are not affected by the electric field and cannot enter the skimmer. This method is not effective when helium gas atoms (molecules) are ionized.